Psychologists say that imagining something is often almost (or equally) as powerful as actually doing it.
You can’t build strength, flexibility or position sense in your body just by imagining. But, practicing a piece in your head – imagining every fingering and bowing, either from memory or while reading the written music – can be a very useful practice method.
This technique is very useful when you are tired from playing too much. Give your body – and your emotions – a break and just lie down and do your (mental) practice in bed or on the sofa !! It’s also a very good way to reinforce relaxation (especially the bed part). Without the physical tensions of actually playing, you can “practice” the piece in a state of total relaxation, which is very helpful when the time comes to pick up the instrument.
With mental practice, you can reinforce both your memory and your technical mastery of a piece of music anywhere and at anytime, making use of idle moments in a bus, train, queue etc where you otherwise might just be getting bored or impatient. I do not recommend doing it while driving however: if a difficult (musical) moment coincides with a difficult automotive moment, you may find yourself going under a truck, which will not help your performance at all!
It is astonishing how, even without the instrument, a difficult passage will also be difficult to imagine clearly and fluently in your mind. And how “cleaning it up” in your mind will make it easier to play when you do actually pick up the instrument.
Listening to a recording of a piece while simultaneously “playing along” in your imagination, with or without the music in front of you, is also a sure way to reveal your insecure moments that need more reinforcing.
A well known English cellist used to have a fingerboard in her car. While stuck in traffic jams or at red lights she would “practice” the music she had to play that evening. This is only for VERY busy people – otherwise it would be a sure sign of a compulsive-obsessive practice disorder !!